1. Field of the Invention
This invention relates to foamed polymers having low loss at microwave frequencies.
2. Introduction to the Invention
It is known to use foamed polymers, e.g. polyethylene, to provide lightweight electrical insulation, e.g. in coaxial cables. For a number of uses, it is also important that the polymer be foamed in order to reduce losses at microwave frequencies. The magnitude of these losses is dependent upon the loss tangent and permittivity of the insulation. By incorporation of voids (which are loss-free ) in the dielectric material, as in foamed polymers, both the loss tangent and permittivity are lowered, yielding a substantial reduction in microwave losses. This result has been achieved in the past by the foaming of low loss polymers, e.g. polyethylene, with conventional blowing agents, e.g. through the decomposition of a chemical such as azobisformamide. Unfortunately, however, these conventional foams lack adequate thermal and/or chemical stability for a number of important uses. For example, a number of end uses require stability at 150.degree. C., and polyethylene foam collapses at such temperatures. Similarly, stability of the foam when exposed to oxygen is often required, and the presence of the antioxidants needed to meet this requirement results in a foam having undesirably high loss.